A combination of 5) and 6) for sure. Right now mesons are overpowered, I'm very happy they are getting an overhaul.
I think mesons should remain viable as a counter against shield only ships and lightly armoured or not armoured ships. So, in regard to the above, they should definitely pass through any amount of shields.
After that, I am ok if they can pass through a FEW armour layers. Perhaps a number of layers equal to the square root of the meson tech level,with a minimum of 1? So meson lvl 1 can pass thorugh 1 layer of armour, meson lvl 4 can pass through 2 levels of armour, and so on.
This would provide great usefulness against unarmoured ships, civilian ships without armour, and shield-exclusive ships. Also, against ships who lost their armour due to other weapons like missiles and such.
In regard to the turret option, I think it should stay because turreted mesons are strictly worse against missiles compared to lasers, railguns and gauss. The role of turreted mesons could be anti-fighter weapon. It would make sense, against fighters.
So all in all my vision for a meson weapon is:
Bypass all shields
Bypass a very modest number of layers of armour
Can be turreted, similar to current balance in VB6 Aurora. Turreted mesons against missiles are worse than railgun (which have more shots), lasers ( which have more range), and gauss (which have more everything but range))
Mission of mesons would be:
Anti fighter
Anti shield exclusive ships
Anti armourless ships or ships who lost their armour